This invention relates to the testing of filters designed for removing colloidal suspensions, and more particularly to a system employing non-volatile residue monitoring devices to determine the capture efficiency of such filters.
The ability to detect colloidal silica, including colloidal silica in very fine particulate form (e.g. as small as 2-3 nanometers in diameter), is of critical importance to the semiconductor industry. The fabrication of very large scale integrated (VLSI) circuits involves multiple semiconductor wafer surface processing stages, with each stage typically followed by a washing of the wafer with ultrapure water. Despite the frequency of washings, and the attendant care with which the ultrapure water is monitored, colloidal silica and other impurities can accumulate on the wafer, leading to defects in the resulting semiconductor device. Accordingly, there is a need to monitor the ultrapure water for the presence of colloidal silica and to remove the colloidal silica by filtration. To the extent practicable, it is desirable to filter the colloidal silica and measure colloidal silica concentrations in situ, i.e. within the water stream as the ultrapure water is supplied to the wafer for cleaning or other treatment.
Colloidal silica presents difficulty in this regard, as it penetrates most known filters, particularly when in very fine particulate form. Such colloidal silica cannot be detected by a scanning electron microscope (SEM), but requires a substantially more expensive scanning tunneling microscope. Alternatively, colloidal silica can be detected by atomic absorption spectrometry to measure a total amount or proportion of silica, with conventional means employed to measure dissolved silica, with colloidal silica then being the total silica less the dissolved silica. Thus, despite known approaches for measuring colloidal silica, for determining filter efficiencies, there remains a need for a low cost and reliable system, operable in situ.
Therefore, it is an object of the present invention to provide a system for determining filter efficiency, in terms of colloidal silica removal, through monitoring a steady state flow of a test liquid including a colloidal suspension.
Another object is to provide a system for determining such efficiency, based upon measuring residue concentrations, regardless of whether the residue consists entirely of a colloidal suspension, or includes other components as well, e.g. dissolved impurities.
A further object of the invention is to provide a process for determining filter efficiencies for removing colloidal silica, without requiring any direct measurement of colloidal silica concentrations.
Yet another object is to provide a system employing one or more non-volatile residue monitoring devices to determine the efficiency at which filters remove colloidal silica suspended in a liquid stream passing through the filter.